The invention relates to a firing aid for the thermal treatment of products consisting of different materials, at firing temperatures of up to 1100° C.
The invention furthermore relates to the production of such a firing aid.
In the thermal treatment of products consisting of different materials at firing temperatures, that is, at very high temperatures of up to 1100° C., and in particular in firing ceramics such as bricks, tiles and other ceramic articles, or in converting starting glasses into glass ceramics, support plates are needed for transporting the products to be fired during the thermal treatment; these are known as firing aids. Stringent demands both thermally and mechanically are made of these firing aids. In particular, they must resist the very high firing temperatures, should have sufficient resistance to temperature shock, also known as spalling resistance, and should have only slight interaction with the product to be fired, or in other words should in particular not give up any particles or cause any scratches or adhesion.
These stringent demands cannot be met, or can be met only with severe limitations, by commercially available material (cordierite, quartz material, Si/SiC).
The essential limitations are the following:                cordierite and Si/SiC cannot be furnished in large enough sizes (such as 160 mm×900 mm);        cordierite is porous and tends to give up particles;        cordierite and Si/SiC support plates can reduce the impact strength of glass ceramic ceramicized on them very sharply;        the low spalling resistance of cordierite and Si/SiC, particularly in relatively large sizes, drastically reduce the heating and cooling rates attainable;        the limited mechanical strength of cordierite and (sintered) quartz material, with large sizes, does not permit thicknesses of less than 12 mm and thus allows only poor energy utilization in firing processes;        quartz material, at usage temperatures>1000° C., tends to form local cristobalite, which is associated with disruptions (dents, clamshell defects) in the surface smoothness];        for cordierite, the manufacturer generally guarantees only a limited number of firing operations (<500).        